Metallic substrate with ceramic coating and method for obtaining it

ABSTRACT

Metallic substrate with a ceramic coating obtained by plasma electrolytic oxidation, resistant to degradation by tribocorrosion of light metals and their alloys, in a liquid and/or semi-solid state with a thickness from 10 to 300 μm and resistant to immersion in said light metals and their alloys without compositional modification of the substrate. Method for obtaining a metallic substrate with a ceramic coating by plasma electrolytic oxidation in which said substrate comprises a core and an outer layer of a metal different from that of the core or of an intermetallic compound, said outer layer being obtained by thermal spraying, laser, solid or liquid diffusion, hot galvanising or cementing. This allows obtaining a ceramic coating applicable to any metallic substrate and resistant to degradation by tribocorrosion.

OBJECT OF THE INVENTION

The present invention lies in the field of electrolytic coatings.

The invention relates to a metallic substrate with a ceramic coatingobtained by plasma electrolytic oxidation, resistant to degradation bytribocorrosion of light metals and their alloys, in a liquid and/orsemisolid state with a thickness from 10 to 300 μm and resistant toimmersion in said light metals and their alloys without compositionalmodification of the substrate.

The invention also relates to a method for obtaining a metallicsubstrate with a ceramic coating by plasma electrolytic oxidationwherein said substrate comprises a metallic core and an outer layer of ametal different from that of the core or an intermetallic compound, saidouter layer being obtained by thermal spraying, laser, diffusion, hotgalvanising or cementing.

BACKGROUND OF THE INVENTION

There exist numerous applications in the casting industry, specificallyfor light alloys such as those of aluminium, in which the material in amolten or semisolid state is in contact with a support metal thatsuffers tribocorrosion, this is, a degradation due to a combination ofcorrosion and wear, that renders it unusable in a short or medium term.

Examples of applications in which a metal suffers the effect of moltenaluminium are rotors, refractory oven walls, casings of immersion heaterresistors, pistons and injection chambers of metals and manufacturing ofcompounds with a metallic or polymer matrix.

Consequently, there is a need to provide the metallic materials with acoating that withstands the aforementioned degradation due totribocorrosion.

The use is known of ceramic materials in relation to molten metals, suchas in heaters and thermocouple sleeves. These materials are veryexpensive and are highly inefficient in transferring heat due to the airpresent between the heating and measuring elements and the tube, as wellas to their very low heat conductivity. However, their greatest drawbackis the highly fragile nature and lack of tenacity of ceramic materials.

Ceramic coatings are known that are attached to the part to protectwhich partially resist degradation by the use of relatively largethickness and for a relatively short period; in addition, de to thetechnique used in their manufacture they cannot be used in parts withcomplex shapes, so that their use in areas with orifices, notches, lips,etc. is entirely excluded and they are recommended only for exposedsurfaces and relatively small dimensions. Another of their drawbacks isthe difficulty in controlling the homogeneity of the layer thicknessthroughout their extension, exhibiting high porosity and thicknessvariation.

Patent EP1231299 discloses a coating on a nonferrous substrate in whicha series of functional compounds are introduced on a porous startinglayer.

The drawback of this coating is that it does not allow using ferroussubstrates, which are common in the industry, as said ferrous substrateshave outstanding mechanical performance and a low cost, instead using assubstrate alloys of light metals (Al, Mg, . . . ) which cannot be usedin accordance with the invention of said patent, as the core of thecoating would degrade when subjected to temperatures very near itsmelting point. In addition, this process is directly intended to achievean open surface porosity that ensures the possibility of introducinglubricating elements. This porosity, in some cases greater than 30%,would be deleterious for use with molten light metals, as it wouldenhance the corrosion process by allowing the molten metal to penetrateto the core.

Patent US20120177837 discloses a coating on a metallic substrate madefrom ceramic powder mixed with metal, this mixture being applied on saidsubstrate.

The drawback of this coating is that it is intended for manufacturing animmersion resistor that is submerged in a static metal bath and has avery simple shape. The technique used to coat the metallic substratewith a ceramic powder having a complex formulation is quite costly dueto its complexity, and in addition it cannot be used for complex shapes.It would not be possible to use it for a great number of components andshapes that are currently used in the casting industry, so that itsapplication is limited exclusively to that for which it was designed,namely an immersion heater.

As shown above, the form in which the coating is obtained affects itsproperties, this is, the method used to obtain the coating is related tothe properties that it will have.

To overcome the aforementioned drawbacks of the state of the art, thefollowing invention is proposed for a metallic substrate with a ceramiccoating and the method for obtaining it.

DESCRIPTION OF THE INVENTION

The present invention is established and characterised in theindependent claims, while the dependent claims describe additionalcharacteristics thereof.

The subject matter of the invention is a metallic substrate with aceramic coating, wherein the metallic substrate is of any type and thecoating is resistant to degradation by tribocorrosion of light metalsand their alloys, in liquid and/or semi-solid state, applicable to anypart of any size, however small, and to any place of the part, howeverinternal.

The technical problem to solve is obtaining said metallic substrate witha ceramic coating having ideal properties regarding tribocorrosion oflight metals and their alloys, in liquid and/or semi-solid state, thatallows implementing it in any type of part.

In view of the above, the present invention relates to a metallicsubstrate with a ceramic coating obtained by plasma electrolyticoxidation, resistant to degradation by tribocorrosion of light metalsand their alloys, in a liquid and/or semi-solid state with a thicknessfrom 10 to 300 μm and resistant to immersion in said light metals andtheir alloys without compositional modification of the substrate.Particularly, it is verified that a specific time value for theresistance to immersion of the coating 81) is 1000 hours.

Optionally, the metallic substrate is a single metal, which can betitanium or zirconium.

In another option the metallic substrate comprises a metallic core andan outer layer of a metal different from that of the core or anintermetallic compound, wherein the core can be made of steel, nickel,titanium, zirconium, refractory metals (Cr, Co, Nb, Mo, W, . . . ) orany of the alloys thereof, and the outer layer is made of anintermetallic compound of titanium and aluminium. The outer layer of ametal different from that of the core is normally obtained by thermalspraying, laser, hot galvanising or cementing, while the outer layer ofan intermetallic compound is normally obtained by spraying or solid orliquid diffusion.

The ceramic coating on any substrate has been verified to present ahardness from 100 to 2000 HV, an adherence from 2 to 30 MPa, and a meanroughness, Ra, from 1 to 5 μm. Optionally, these parameters can beimproved by physical operations such as polishing, machining, etc. asthe applied coating withstands physical operations applied to it.

The coating provides the assembly with the substrate with the advantagesof tenacity, hot mechanical strength, resistance to thermal shock andthe conductivity of a metallic material.

Another advantage is that the substrate with the coating can replaceceramic materials, which are fragile and expensive, in direct contactwith the molten metal that withstands tribocorrosion.

A further advantage is that the substrate is reusable, and the ceramiccoating can be generated on it as many times as needed, preventingwasting a large amount of metallic material and making the processrecyclable and low in cost.

The invention also relates to a method for obtaining a metallicsubstrate with a ceramic coating by plasma electrolytic oxidationwherein said substrate comprises a core and an outer layer of a metaldifferent from that of the core or an intermetallic compound, said outerlayer being obtained by thermal spraying, laser, solid or liquiddiffusion, hot galvanising or cementing.

Preferably, the thermal spraying, laser, hot galvanising and cementingis performed on a core when the coating is obtained as an outer layer ofa metal different from that of the core, while spraying and solid orliquid diffusion is performed on a core when the coating is obtained asan outer layer of an intermetallic compound.

One advantage of the method is that is allows obtaining a coating with agreat resistance, particularly on steels, as it includes an intermediatemetallic substrate or an intermetallic layer using known techniques thatare therefore simple and relatively inexpensive.

DESCRIPTION OF THE DRAWINGS

This specification is supplemented with a set of drawings illustratingthe preferred embodiment, which are never intended to limit theinvention.

FIG. 1 represents a SEM image of the coating after being subjected to aperformance test in molten aluminium in static conditions, showing thecoating layer on a metallic substrate which has not suffered any attack.

FIG. 2 represents a SEM image of the coating after being subjected to aperformance test for forced tribocorrosion in molten aluminium, showingthe molten aluminium layer that bathed the coating and the coating layeron a metallic substrate which has not suffered any attack.

FIG. 3 represents an assembly used for the forced tribocorrosion test inwhich a motor turns a shaft from which hangs the sample while it isintroduced in a crucible filled with light metal in liquid or semisolidstate, the shaft turning in any one of the two senses, clockwise oranticlockwise.

FIG. 4 represents an assembly used for the forced tribocorrosion test inwhich a motor turns a shaft with a carrousel from which hang the sampleswhile they are introduced in a crucible filled with light metal inliquid or semisolid state, the shaft alternatively turning in one senseand then the opposite sense, in order to increase the wear induced bythe process.

PREFERRED EMBODIMENT OF THE INVENTION

To obtain the metallic substrate (2) with a ceramic coating (1) that isthe subject matter of the invention, a surface preparation is performedfor the metallic substrate (2) as follows:

-   -   Stripping in an acidic solution adapted to the base material        being treated;    -   Working area delimited by resin;    -   Electrical contact with a copper or aluminium wire.

The plasma electrolytic oxidation (PEO) coating (1) is performed asfollows:

a) Apparatus: power supply, control and data acquisition card;

b) Electrochemical cell: reactor with stirring in thermostatic sleeve,cathode AISI 316L;

c) Aqueous alkaline electrolyte, specific for each of the metallicsubstrates (2) to be coated at a temperature of up to 60° C.;

d) Treatment (0-10000 s): pulsed signal frequency of 50-4000 Hz, currentdensity 5-800 mA/cm.

The voltage records show values from 10 to 1000 V.

This allows obtaining a thickness for the coating (1) from 10 to 300 μm,according to measurements as per ISO 2360.

The coating (1) is subjected to multiple tests for resistance totribocorrosion of light metals in liquid and semi-solid state, both instationary conditions in which it is simply submerged in the metal andin dynamic conditions, using tests of 1000 h in light alloy at differenttemperatures, preferably 590° C., 650° C. and 750° C. FIG. 1 shows anenlarged view of a coating (1) on its substrate (2) after beingsubjected to a performance test in molten aluminium in staticconditions.

The dynamic conditions are obtained by stirring in axial mode, FIG. 3,and in carrousel, FIG. 4, showing that the coating (1) remains intact atthe end of the test period, FIG. 2.

In the forced tribocorrosion test with stirring in axial mode, FIG. 3, amotor (4) turns a shaft (5) from which hangs the sample (6) while it isintroduced in a crucible (7) filled with light metal (8) in liquid orsemisolid state, the shaft (5) turning in any one of the two senses,clockwise or anticlockwise.

In the forced tribocorrosion test with stirring in carrousel mode, FIG.4, the motor (4) turns a shaft (5) with a carrousel (9) from which hangthe samples (6) while they are introduced in a crucible (7) filled withlight metal (8) in liquid or semisolid state, the shaft (5)alternatively turning in one sense and then the opposite sense, in orderto increase the wear induced by the process.

Multiple tests were carried out to characterize the material, changingthe time of permanence inside the molten metal, increasing it (24 hours,48 hours, 96 hours . . . ) to show the resistance of the materialwithout compositional variation of the substrate for at least 1000hours.

FIG. 2 represents an enlarge view of a coating (1) on its substrate (2)after being subjected to a performance test for forced tribocorrosion inmolten aluminium, showing the molten aluminium layer (3) that bathed thecoating (1), revealing that the coating (1) did not suffer any attackafter 100 hours of testing.

The substrate (2) on which the coating (1) is obtained is made of asingle metal or a core and an outer layer of a different metal than thatof the core or an intermetallic compound, with similar test results forany of these configurations.

The coating (1) has a hardness from 100 to 2000 HV, depending to a greatextent on the material of the substrate (2), as obtained from 10measurements with a load of 10 g and a penetration time of 20 s.

An adherence from 2 to 30 MPa is obtained from 3 measurements with adolly 10 mm in diameter, epoxy adhesive and a uniform traction tensionincrease of 1 MPa/s, as per ISO 4624.

The mean roughness, R_(a), from 1 to 5 μm, and the maximum roughness,under 20 μm, are each obtained from 3 measurements with a roughnesstester and 0.25 mm Gaussian filter, at a distance of 4 mm.

The coefficient of friction from 0.2 to 1″ is obtained from 3measurements using a ball-on-disk tribometer with a load of 2 N,distance 1000 m, 200 rpm (0.08 m/s), turning radius 4 mm, as per ASTMG99-04; as a countersample: WC ball with 6 mm diameter.

1. Metallic substrate with a ceramic coating obtained by plasmaelectrolytic oxidation, resistant to degradation by tribocorrosion oflight metals and their alloys, in a liquid and/or semi-solid state,characterised in that said coating (1) has a thickness from 10 to 300 μmand is resistant to immersion in said light metals and their alloyswithout compositional modification of the substrate (2).
 2. Metallicsubstrate with a ceramic coating according to claim 1, wherein themetallic substrate (2) is a single metal.
 3. Metallic substrate with aceramic coating according to claim 1, wherein the metal is titanium orzirconium.
 4. Metallic substrate with a ceramic coating according toclaim 1, wherein the metallic substrate (2) comprises a metallic coreand an outer layer of a metal different from that of the core, saidlayer being obtained by thermal spraying, hot galvanising or cementing.5. Metallic substrate with a ceramic coating according to claim 1,wherein the metallic substrate (2) comprises a metallic core and anouter layer of an intermetallic compound, said layer being obtained byspraying or by solid or liquid diffusion.
 6. Metallic substrate with aceramic coating according to claim 4, wherein the core is of steel,nickel, titanium, zirconium, refractory materials or any of the alloysthereof.
 7. Metallic substrate with a ceramic coating according to claim5, wherein the core is of steel, nickel, titanium, zirconium, refractorymaterials or any of the alloys thereof.
 8. Metallic substrate with aceramic coating according to claim 4, wherein the outer layer is of alight metal selected from among aluminium, magnesium, titanium andzirconium.
 9. Metallic substrate with a ceramic coating according toclaim 5, wherein the outer layer is of an intermetallic compound oftitanium and aluminium.
 10. Metallic substrate with a ceramic coatingaccording to claim 1, wherein the coating (1) has a hardness from 100 to2000 HV.
 11. Metallic substrate with a ceramic coating according toclaim 2, wherein the coating (1) has a hardness from 100 to 2000 HV. 12.Metallic substrate with a ceramic coating according to claim 3, whereinthe coating (1) has a hardness from 100 to 2000 HV.
 13. Metallicsubstrate with a ceramic coating according to claim 4, wherein thecoating (1) has a hardness from 100 to 2000 HV.
 14. Metallic substratewith a ceramic coating according to claim 5, wherein the coating (1) hasa hardness from 100 to 2000 HV.
 15. Metallic substrate with a ceramiccoating according to claim 6, wherein the coating (1) has a hardnessfrom 100 to 2000 HV.
 16. Metallic substrate with a ceramic coatingaccording to claim 7, wherein the coating (1) has a hardness from 100 to2000 HV.
 17. Metallic substrate with a ceramic coating according toclaim 8, wherein the coating (1) has a hardness from 100 to 2000 HV. 18.Metallic substrate with a ceramic coating according to claim 9, whereinthe coating (1) has a hardness from 100 to 2000 HV.
 19. Metallicsubstrate with a ceramic coating according to claim 1, wherein thecoating (1) has an adherence from 2 to 30 MPa.
 20. Metallic substratewith a ceramic coating according to claim 2, wherein the coating (1) hasan adherence from 2 to 30 MPa.
 21. Metallic substrate with a ceramiccoating according to claim 3, wherein the coating (1) has an adherencefrom 2 to 30 MPa.
 22. Metallic substrate with a ceramic coatingaccording to claim 4, wherein the coating (1) has an adherence from 2 to30 MPa.
 23. Metallic substrate with a ceramic coating according to claim5, wherein the coating (1) has an adherence from 2 to 30 MPa. 24.Metallic substrate with a ceramic coating according to claim 6, whereinthe coating (1) has an adherence from 2 to 30 MPa.
 25. Metallicsubstrate with a ceramic coating according to claim 7, wherein thecoating (1) has an adherence from 2 to 30 MPa.
 26. Metallic substratewith a ceramic coating according to claim 8, wherein the coating (1) hasan adherence from 2 to 30 MPa.
 27. Metallic substrate with a ceramiccoating according to claim 9, wherein the coating (1) has an adherencefrom 2 to 30 MPa.
 28. Metallic substrate with a ceramic coatingaccording to claim 1, wherein the coating (1) has a mean roughness,R_(a), from 1 to 5 μm.
 29. Metallic substrate with a ceramic coatingaccording to claim 2, wherein the coating (1) has a mean roughness,R_(a), from 1 to 5 μm.
 30. Metallic substrate with a ceramic coatingaccording to claim 3, wherein the coating (1) has a mean roughness,R_(a), from 1 to 5 μm.
 31. Metallic substrate with a ceramic coatingaccording to claim 4, wherein the coating (1) has a mean roughness,R_(a), from 1 to 5 μm.
 32. Metallic substrate with a ceramic coatingaccording to claim 5, wherein the coating (1) has a mean roughness,R_(a), from 1 to 5 μm.
 33. Metallic substrate with a ceramic coatingaccording to claim 6, wherein the coating (1) has a mean roughness,R_(a), from 1 to 5 μm.
 34. Metallic substrate with a ceramic coatingaccording to claim 7, wherein the coating (1) has a mean roughness,R_(a), from 1 to 5 μm.
 35. Metallic substrate with a ceramic coatingaccording to claim 8, wherein the coating (1) has a mean roughness,R_(a), from 1 to 5 μm.
 36. Metallic substrate with a ceramic coatingaccording to claim 9, wherein the coating (1) has a mean roughness,R_(a), from 1 to 5 μm.
 37. Method for obtaining a metallic substrate (2)with a ceramic coating (1) by plasma electrolytic oxidationcharacterised in that said substrate (2) comprises a core and an outerlayer of a metal different from that of the core or of an intermetalliccompound, said outer layer being obtained by thermal spraying, laser,solid or liquid diffusion, hot galvanising or cementing.
 38. Methodaccording to claim 12, wherein the thermal spraying, laser, hotgalvanising and cementing is performed on a core when the coating (1) isobtained as an outer layer of a metal different from that of the core.39. Method according to claim 12, wherein the spraying and solid orliquid diffusion is performed on a core when the coating (1) is obtainedas an outer layer of an intermetallic compound.